In modern papermaking, an ever-increasing emphasis is being placed on more complete recovery of furnish materials, such as pigments, fibers and additives, as a part of the ultimate sheet. Some of the many benefits the papermaker realizes by increasing the retention, especially of the fine particulate matter associated with respect to these components, are:
(a) better economic utilization of the furnish materials. Obvious economic advantages are achieved by increasing the retention of the furnish materials on the wire of the paper-making machine to provide more paper product and less waste;
(b) improved quality of the resultant sheet product. The retention of supplied furnish material yields a more uniform grade of product and fewer rejects on the production end product;
(c) less down-time due to a cleaner system. The retenton of most of the fiber, fines, fillers and additives on the web does not allow these materials to accumulate in the recirculating white water of the production system. In systems where the white water is at least partially recirculated, this provides less build-up or concentration of fines in the head box. It also provides less solids in the effluent stream and thereby minimizes the need for extensive treatment of the stream before release;
(d) increased production. By providing a means to retain the solids on the web and provide an increased drainage rate, a paper making machine can be frequently increased in rate and thereby increase production; and
(e) more versatile paper production. The ability to retain more solids on the web can often reduce two-sidedness of the paper product and provides a means of incorporating materials otherwise impossible to use.
A large variety of materials have been used as retention aids. The most widespread of these, and among the oldest in use, are the salts of aluminum, in particular aluminum sulfate, sodium aluminate and sodium phosphoaluminate. These materials, however, have the defects of being required to be used in large amounts and are known not to be a highly efficient retention aid.
Various polymeric materials, from natural occurring gums to synthetic resins have also been used as retention aids. These materials include natural and synthetic starches, anionic polyelectrolytes, such as partially hydrolyzed polyacrylamides, and cationic polymers such as polyamidoamines and polyalkyleneimines. Polyethyleneimine and, to a lesser extent, polypropyleneimine are used in the papermaking industry as retention aids for fillers and fibers, as aids for improving the drainage rate during sheet formation and as flocculants in pulp recovery. Such alkyleneimine type polymers have been known and used for a long time. More recently these polymers have been extended with epihalohydrins (See German Patent No. 1670296) or with epihalohydrin capped polyalkylene oxides (See German Published Application No. 1546290 and U.S. Pat. No. 4,066,494). The preferred materials have been those having high degrees of alkyleneimine units in the polymer.
It is also known that polyamidoamines formed from polyamines with dicarboxylic acids and subsequently crosslinked with epihalohydrin provide a good fiber retention aids. Examples of such crosslinked polyamidoamines are described in U.S. Pat. Nos. 3,250,664 and 3,893,885.
The polyalkyleneimine and polyamidoamine products described above each suffer from the drawback of being effective within a limited pH range. The former material is effective at neutral and alkaline pHs while the latter material is best used under acid pH conditions. In the manufacture of paper, the pH will vary depending on the particular type of paper being formed. For this reason it is highly desirable to have a retention agent which is capable of being used under both neutral and acid pH conditions. This requirement led to the formation of a polyethyleneimine grafted polyamidoamine copolymer, as described in U.S. Pat. No. 3,642,572. The polyamidoamine is initially formed and then an aziridine compound is grafted onto the backbone by standard cationic polymerization. Although the resultant product is useful at a wide variety of pH conditions, the product has the drawback of being formed from highly toxic aziridine compounds. The use of such monomers require special handling and apparatus which greatly increases the cost of the finished product and, indirectly, the cost of the paper product.
Another means of attempting to achieve a product which is effective at a variety of pH conditions is to couple together polyamines and polyamidoamines with the aid of a poly functional compound as described in U.S. Pat. No. 4,250,299. Although the polyamidoamines have moderate molecular weight, the polyamines are generally low molecular weight material. The polyfunctional compounds taught useful are those capable of reacting with secondary and/or tertiary amino groups present in the polyamidoamine (by its formation from a dicarboxylic acid with an amine required to have at least two primary and at least one secondary and/or tertiary amino group) and with such groups present in the polyamine. Although this process alleviates the need for aziridine as a means to provide alkyleneimine polymeric segments in the final product (as described in U.S. Pat. No. 3,642,572), it produces high percentages of low molecular weight material as part of the final product. Such low molecular weight material is formed by the preferential coupling (due to high molar ratio of reactive amino groups in the polyamine in comparison to the low presence of such groups in the polyamidoamine) of low molecular weight polyamines together. Even where there is sequential addition of the components, the polymeric product still has a high percentage of low molecular weight material. Products formed according to '299 have been found to be less effective on a dosage weight basis than other known retention and drainage agents, such as those of '572. This is probably due to the high percentage of low molecular weight material present in the former material.
It is highly desired to have a material which is capable of exhibiting a high degree of retention of fibers, fillers, and pigments in the manufacture of paper, for accelerating the dehydration of paper raw materials, and for working-up waste water from paper machines. The desired material must be capable of exhibiting these properties over a wide variety of pH conditions, be capable of being formed from materials easily handled and obtained, and be capable of being of a substantially high molecular weight.